Lipid Layers of the Cell Membrane

Lipid Layers of the Cell Membrane

The central lipid layer is a bilayered structure. This is formed by a thin film of lipids. The characteristic feature of lipid layer is that, it is fluid in nature and not a solid structure. So, the portions of the membrane move from one point to another point along the surface of the cell. The materials dissolved in lipid layer also move to all areas of the cell membrane.

Major lipids are:

1. Phospholipids

2. Cholesterol.

1. Phospholipids

Phospholipids are the lipid substances containing phosphorus and fatty acids. Aminophospholipids, sphingo -myelins, phosphatidylcholine, phosphatidyletholamine, phosphatidylglycerol, phosphatidylserine and phospha tidylinositol are the phospholipids present in lipid layer of cell membrane. Phospholipid molecules are arranged in two layers. Each phospholipid molecule resembles the headed pin in shape. The outer part of the phospholipid molecule is called the head portion and the inner portion is called the tail portion. Head portion is the polar end and it is soluble in water and has strong affinity for water (hydrophilic). Tail portion is the non-polar end. It is insoluble in water and

repelled by water (hydrophobic). Two layers of phospholipids are arranged in such a way that the hydrophobic tail portions meet in the center of the membrane. Hydrophilic head portions of outer

layer face the ECF and those of the inner layer face ICF (cytoplasm).

2. Cholesterol

Cholesterol molecules are arranged in between the phospholipid molecules. Phospholipids are soft and

oily structures and cholesterol helps to ‘pack’ the phospholipids in the membrane. So, cholesterol is

responsible for the structural integrity of lipid layer of the cell membrane.

Functions of Lipid Layer in Cell Membrane

Lipid layer of the cell membrane is a semipermeable membrane and allows only the fat-soluble substances to pass through it. Thus, the fat-soluble substances like oxygen, carbon dioxide and alcohol can pass through this lipid layer. The water-soluble substances such as glucose, urea and electrolytes cannot pass through this layer.

Protein Layers of the Cell Membrane

Protein layers of the cell membrane are electron-dense layers. These layers cover the two surfaces of the

central lipid layer. Protein layers give protection to the central lipid layer. The protein substances present in these layers are mostly glycoproteins.

Protein molecules are classified into two categories:

1. Integral proteins or transmembrane proteins.

2. Peripheral proteins or peripheral membrane proteins.

1. Integral proteins

Integral or transmembrane proteins are the proteins that pass through entire thickness of cell membrane from one side to the other side. These proteins are tightly bound with the cell membrane.

Examples of integral protein:

i. Cell adhesion proteins

ii. Cell junction proteins

iii. Some carrier (transport) proteins

iv. Channel proteins

v. Some hormone receptors

vi. Antigens

vii. Some enzymes.

2. Peripheral proteins

Peripheral proteins or peripheral membrane proteins are the proteins which are partially embedded in the

outer and inner surfaces of the cell membrane and do not penetrate the cell membrane. Peripheral proteins are loosely bound with integral proteins or lipid layer of cell membrane. So, these protein molecules dissociate readily from the cell membrane.

Examples of peripheral proteins:

i. Proteins of cytoskeleton

ii. Some carrier (transport) proteins

iii. Some enzymes.

Functions of Proteins in Cell Membrane

1. Integral proteins provide the structural integrity of the cell membrane

2. Channel proteins help in the diffusion of water soluble substances like glucose and electrolytes

3. Carrier or transport proteins help in the transport of substances across the cell membrane by means of

active or passive transport

4. Pump: Some carrier proteins act as pumps, by which ions are transported actively across the cell

membrane

5. Receptor proteins serve as the receptor sites for hormones and neurotransmitters

6. Enzymes: Some of the protein molecules form the enzymes and control chemical (metabolic) reactions

within the cell membrane

7. Antigens: Some proteins act as antigens and induce the process of antibody formation

8. Cell adhesion molecules or the integral proteins are responsible for attachment of cells to their neighbors or to basal lamina.

Carbohydrates of the Cell Membrane

Some of the carbohydrate molecules present in cell membrane are attached to proteins and form

glycoproteins (proteoglycans). Some carbohydrate molecules are attached to lipids and form glycolipids.

Carbohydrate molecules form a thin and loose covering over the entire surface of the cell membrane

called glycocalyx.

Functions of Carbohydrates in Cell Membrance

1. Carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell

2. Glycocalyx from the neighboring cells helps in the tight fixation of cells with one another

3. Some carbohydrate molecules function as the receptors for some hormones.

FUNCTIONS OF CELL MEMBRANE

1. Protective function: Cell membrane protects the cytoplasm and the organelles present in the cytoplasm

2. Selective permeability: Cell membrane acts as a semipermeable membrane, which allows only some

substances to pass through it and acts as a barrier for other substances

3. Absorptive function: Nutrients are absorbed into the cell through the cell membrane

4. Excretory function: Metabolites and other waste products from the cell are excreted out through the

cell membrane

5. Exchange of gases: Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane

6. Maintenance of shape and size of the cell: Cell membrane is responsible for the maintenance of shape

and size of the cell.

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